An office machine such as a printer or a scanner is widely used in the office. For achieving diversified functions and integrating more functions, the office machine is usually equipped with a post-processing device (e.g. a stapler). By the stapler, plural documents outputted from the printer or the scanner can be automatically stapled in order to enhance the working efficiency. The operations of the post-processing device (e.g. a stapler) will be illustrated as follows. Firstly, the documents are placed on a sheet placement tray. Then, the edges of these documents are aligned with each other by a sheet alignment device. Then, a stapling operation is performed by the stapler, so that the corresponding parts of these documents are combined together. Afterwards, the stapled documents are ejected from the sheet placement tray to the outer portion of the office machine through a sheet ejection device. In other words, it is necessary to install the sheet alignment device to align the document with each other and install the sheet ejection to automatically eject the stapled documents.
Please refer to FIGS. 1A˜1D. FIG. 1A is a schematic perspective view illustrating a sheet alignment device of a conventional sheet alignment and ejection mechanism. FIG. 1B is a schematic perspective view illustrating the conventional sheet alignment and ejection mechanism applied to an office machine. FIG. 1C is a schematic perspective view illustrating a sheet ejection device of the conventional sheet alignment and ejection mechanism, in which the sheet ejection device is in a standby status. FIG. 1D is a schematic perspective view illustrating a sheet ejection device of the conventional sheet alignment and ejection mechanism, in which the sheet ejection device is in a sheet-ejecting status. The conventional sheet alignment and ejection mechanism shown in FIGS. 1A-1D is disclosed in for example U.S. Pat. No. 7,823,868.
The conventional sheet alignment and ejection mechanism 1 comprises a sheet alignment device 11, a sheet ejection device 12, a sheet placement tray 13 and a post-processing unit (not shown). As shown in FIGS. 1A and 1B, the sheet alignment device 11 comprises plural paddles 11a, a first rotation shaft 11b and a first power device 11c. As shown in FIGS. 1C and 1D, the sheet ejection device 12 comprises a pushing part 12a, a ring-shaped conveyor belt 12b, a roller 12c and a second power device 12d. The sheet placement tray 13 comprises plural supporting arms 13a. The post-processing unit is for example a stapler.
Please refer to FIG. 1B. The first rotation shaft 11b is disposed above the sheet placement tray 13. The paddles 11a are sheathed around the first rotation shaft 11b. The first power device 11c is connected with the first rotation shaft 11b. Please refer to FIGS. 1C and 1D. The roller 12c is disposed under the sheet placement tray 13. The pushing part 12a is protruded from an outer surface of the ring-shaped conveyor belt 12b. The ring-shaped conveyor belt 12b is sheathed around the outer periphery of the roller 12c to enclose the sheet placement tray 13. The second power device 12d is connected with the roller 12c. 
Hereinafter, the operations of the conventional sheet alignment and ejection mechanism will be illustrated with reference to FIGS. 1A-1D and also FIG. 1E. FIG. 1E is a schematic side view illustrating the conventional sheet alignment and ejection mechanism. Firstly, the sheet alignment device 11 and the sheet ejection device 12 are located at the home positions. As shown in the drawings, the paddles 11a of the sheet alignment device 11 are oriented upwardly and distant from the sheet placement tray 13, and the pushing part 12a is disposed under the roller 12c. When a sheet S is ready to be introduced to the sheet placement tray 13, the sheet S firstly falls down to the paddles 11a. Then, the first rotation shaft 11b and the paddles 11a are driven by the first power device 11c to be rotated in the direction d. As the paddles 11a are rotated, the sheet S falls down to the sheet placement tray 13, and then the sheet S is leant against a supporting arm 13a of the sheet placement tray 13. The first rotation shaft 11b and the paddles 11a are continuously rotated in the direction d until plural sheets S are leant against the supporting arm 13a of the sheet placement tray 13 and aligned with each other.
After the sheet alignment operation is completed, the plural sheets S on the sheet placement tray 13 are stapled by the post-processing unit. Then, the roller 12c is driven by the second power device 12d to be rotated in the direction d, and thus the ring-shaped conveyor belt 12b is rotated in the direction d. Consequently, the pushing part 12a is moved upwardly from the position under the roller 12c to the sheet placement tray 13. The pushing part 12a is linearly moved on the surface of the sheet placement tray 13 until the stapled sheets S are ejected out of the sheet placement tray 13.
From the above discussions, in the conventional sheet alignment and ejection mechanism 1, the operations of the sheet alignment device 11 and the sheet ejection device 12 are respectively controlled by the first power device 11c and the second power device 12d. 
However, the conventional sheet alignment and ejection mechanism 1 still has some drawbacks. As previously described, since two power devices are employed to control the sheet alignment device and the sheet ejection device, a bulky internal space of the office machine is necessary. Under this circumstance, the fabricating cost of the office machine is increased.
Therefore, there is a need of providing an improved sheet alignment and ejection mechanism so as to obviate the drawbacks encountered from the prior art.